DIY Rear Projection TV

[faithblinded]

I'd be interested to see what "cheap" zoom projection lensees you are talking about.
All the zoom lenses I've seen "cheap" on eBay have been of a far too small diameter to capture an entire 15" screen.
Furthermore, these small zooms are generally meant to be within an inch of the image source, which was usually about the perfect size for the lens, i.e. 9mm film etc.
Don't let me discourage you though, lets see what lens you're talking about.

[jrhilton]

far too small diameter to capture an entire 15" screen

I'm talking about projection lenses, they don't need to cover the whole screen as they are designed to project a source which is converging/ has aready converged. The are designed to be a certain distance from the focal point of the condensor element of the projector that is before the film/slide ect not the film it's self. As an OHP has a Fresnel Lens the light will be converging to a focal point so you can use a projection lens.

these small zooms are generally meant to be within an inch of the image source

You could still use these if you have a condensor lens with a long focal length behind the panel, e.g the fresnel lens. Though you want to use a lens that has the widest aperture you can get hold of so the image is as bright as possible, so an 8mm or 16mm film lens would be not be the best bet.

As for ebay,for example I have seen projection lenses for medium format slide projectors (6x6cm slides) go for less than $20 USDs so they are cheap compared to what they cost new. These would be great because many were designed for projecting images from a meter or so for close copy work to 25 meters+ for public viewing. I have even seen a lens that was said to be from a Gaumont 70mm cinema projector, (not that this would be of any use here) but it is stunning what is out on Ebay. Maybe I should have been more clear as to what sort of lens I was talking about in my origional post. I did not mean the type that you will have in a RPTV in front of the crt ect which only project what is just behind them. There certainally are lots of projection lenses out there for 35mm + film sizes, i've seen enough in all my years of working with film, lots must still be out there.

Hope this helps.

[Hezz]

JR,

The fact that you are collapsing the light field still will not allow you to use one of those typical projection lenses unless they have the correct focal length and a field of view that works for the LCD's size.

I do not know what the typical focal length of one of those lenses are but it would have to be between 325 - 500 mm to work with a 15 inch LCD. And even if it had the right focal length if it was designed to image a smaller object it will not have adequate field of view for a large LCD.

If the objective lens system will not image the object without any additional aids the lens will not work. All of the other light engine light manipulations are only to increase the light through the system.

Hezz

[rmccoll]

Well I got my 1.8inch Lilliput LCD today and to my delight I can use it the controller board is completely out of the way. The resolution on this thing is amazing I see can the smallest words on the screen. The resolution is 116160 which is about the same as the Sony Playstion LCD.
I plan to use a 75 to 100 watt par16 halogen bulb as my light source I may go even higher, the diameter of the lense is 2 inches so I will cut out the condenser lense and put the bulb right behind the lcd, with a piece of ground glass between them. I will use a 80mm fan to cool the whole thing since it's so small. I have not figure what projection lense I'm going to use. When I get my camera back I will post pictures.

[ebswift]

Well I got a phone call from "Rosco" yesterday, the price for rear projection screen material in Australia is $48 p/m at 55" wide, and $25 postage. That seems a reasonable price since only about 2m is necessary. There is a choice between the grey and dark grey screen, so I might see if I can get a small sample of each because I don't want to be buying the wrong screen type .

Anyway, I'll keep pricing bits and pieces and getting ideas out of this forum for optics etc. I still have no real idea about all the requirements. The guy from Rosco said that distance would be an issue with RP, but I explained to him that I believe it can be done with optics according to what I've read here. Hopefully it won't be too long before I can actually start buying some bits.

[dbc105]

Due to work I have not been able to do anything on my project.

ebswift, it is going to be interesting to see what results you get with the screen material. Is this material smooth on both sides and look like the screens on the newer HDTV or does it have ridges like on the older style RPTV's? What is the material called? That might help in locating it here in the states.

rmccoll, I have seen resolutions numbers like that before but how does that relate to like 800 x 600 on a pc monitor? I am sure it is simple math, I just don't know what it is.

I still like the simplicity of the OHP inside the box directing toward the rear of the screen. I wonder how well the lilliput that ace is using in his projector would do on an OHP? Any ideas?

David

[ebswift]

dbc105, Rosco seems to be worldwide, if you go to their home page (http://www.rosco.com) it shows a world map where you select your home country.

I haven't seen the actual material yet, and to be honest I'm not even sure if I'm going to run for this idea at all, I'm still researching. The only thing that would prevent me from going this way is if the resulting image would be inferior to making something that's front projection. I did read somewhere in these forums that the image is inferior, but I really find that hard to believe when you're taking ambient light into consideration. Even in a dark environment, rear projection really should equal or better front projection(?). Maybe someone who has actually experimented with both types can tell us some more.

[dbc105]

ebswift, thanks for the link. In their web site I found a page that would be useful to everyone here, front or rear projection.

http://www.rosco-ca.com/technotes/sc...tbcs.html#REAR

This page has a lot of good screen info.

David

[ace3000_1]

Anyway, I'll keep pricing bits and pieces and getting ideas out of this forum for optics etc. I still have no real idea about all the requirements. The guy from Rosco said that distance would be an issue with RP, but I explained to him that I believe it can be done with optics according to what I've read here. Hopefully it won't be too long before I can actually start buying some bits.

heya ebswift, nice find u got going there on the screen, and yep we will and can nut this rear projection out, just a matter of finding a good screen and the right optics, for some parts i can supply a few things like reflectors ect for the light engine, they are hard to find normally and cost a mint to buy, i get them from a wholesaller so i get them cheap, this enables me to sell them cheap and easily aquairable (if thats a word lol) keeping our project to a minimal cost, but yeah i must say u guys in here are starting to move ahead in here, im not doing rear projection just yet, but soon though, and its good to see a good bunch of guys in this feild already onto it, ill be in and out from time to time, good luck.

Trev

[dbc105]

Well I have been tied up for the last week or so. My daughter crashed her car so me and the wife have been having to get her to school and work and also been looking for a replacement car so I have had little time. Has anyone made any progress in the last couple of weeks?

David